1. Field of the Invention
The present invention relates to a suction pad for attracting under suction a disk-shaped workpiece having a central hole, such as a mini disc, a compact disc, a laser disc, or the like and carrying the attracted disk-shaped workpiece to a desired position.
2. Description of the Related Art
Suction pads connected to a source of vacuum have heretofore been widely used to attract and carry workpieces. Such a suction pad comprises a main body connected to a source of vacuum through a tube or the like mounted in a port, and a flexible attracting portion integral with the main body for contacting a surface of a workpiece when the workpiece is to be attracted by the suction pad.
One known suction pad for attracting under suction a substantially disk-shaped workpiece having a central hole is disclosed in Japanese laid-open utility model publications Nos. 61-169541 and 62-172582, for example.
The suction pad disclosed in Japanese laid-open utility model publication No. 61-169541 is shown in FIG. 13A of the accompanying drawings. As shown in FIG. 13A, the suction pad, generally designated by the reference numeral 2, comprises a larger-diameter pad 6 in the form of a substantially circular dome which is held in the bottom of a housing 4, and a smaller-diameter hermetic seal ring 10 supported coaxially with the housing 4 by a centering shaft 8. Each of the larger-diameter pad 6 and the smaller-diameter hermetic seal ring 10 serves as a pad for attracting a surface of a workpiece 12 under suction. The larger-diameter pad 6 and the smaller-diameter hermetic seal ring 10 are of a double structure in which they are held coaxially with each other. The hermetic seal ring 10 supports a land 16 on its bottom surface for fitting in a hole 14 defined centrally in a workpiece 12 to position the suction pad 2 with respect to the workpiece 12. For attracting the workpiece 12 under suction, the hermetic seal ring 10 is held against the workpiece 12 along an inner peripheral region around the hole 14 thereby to close the hole 14, and the pad 6 which is larger in diameter than the hermetic seal ring 10 is brought into abutment against the workpiece 12. Then, a negative pressure or vacuum is developed in a space between the hermetic seal ring 10 and the pad 6 for thereby attracting the workpiece 12 under suction.
FIG. 13B of the accompanying drawings shows the suction pad disclosed in Japanese laid-open utility model publication No. 62-172582. As shown in FIG. 13B, the suction pad, generally designated by the reference numeral 18, has a pad support 22 with a vacuum hole 20 defined axially in its center. The suction pad 18 also has a smaller-diameter inner seal 24 fitted in the pad support 22 and a larger-diameter outer seal 26 fitted over the pad support 22. The inner seal 24 and the outer seal 26, each serving as a pad, are of a double structure in which they are held coaxially with each other. In order to attract a workpiece 28 under suction, the inner seal 24 closes a hole 29 defined centrally in the workpiece 28, and the outer seal 26 is held against the workpiece 28 around the inner seal 24. Then, a negative pressure or vacuum is developed in a space between the inner and outer seals 24, 26 for thereby attracting the workpiece 28 under suction.
As described above, the conventional suction pads 2, 18 have two coaxially positioned pads of different diameters for attracting the doughnut-shaped workpieces 12, 28.
For attracting and carrying a doughnut-shaped workpiece having a central hole such as a compact disc, a mini disc, or the like, a suction pad is required to be held against an information-free area of the workpiece, i.e., an inner peripheral region around the central hole or an outer circumferential region of the workpiece. This is because information is recorded in a region between the inner peripheral region around the central hole and the outer circumferential region of the workpiece, and if the suction pad contacted the information-recorded region, it would possibly damage the information-recorded region, making it difficult for a pickup to read the recorded information from that region.
If such a workpiece which has limited pad-contact regions is to be attracted and carried by the conventional suction pads 2, 18, then it is necessary to position the smaller- and larger-diameter pads accurately on the inner peripheral region and the outer circumferential region, respectively. Consequently, the smaller- and larger-diameter pads need to be assembled accurately on the main body of the suction pads, and have to be formed with high dimensional accuracy. Actually, however, the smaller- and larger-diameter pads suffer dimensional errors owing to irregularities introduced when they are manufactured, and such dimensional errors prevent them from being assembled accurately.
In a process of manufacturing a compact disc, a mini disc, or a laser disc, the disc is positioned and held by a support rod which projects though the central hole of the disc. When the suction pads 2, 18 are used to attract the disc which is positioned and held by the support rod, since the support rod presents an obstacle to the suction pads 2, 18, the suction pads 2, 18 find it difficult to attract the disc.